Controlled assembly of nanoparticles into dimers and complex nanoclusters with asymmetric configurations (in terms of geometries and compositions) is of great interest because of their novel properties and possible promising applications (e.g., biosensing, labeling, display, catalysis, etc.). For example, dimers made of either uniform Ag nanoparticles or Au nanoparticles have been demonstrated to be a class of promising surface-enhanced Raman scattering (SERS) substrates due to the creation of hot spots between individual nanoparticles in each dimer. In these dimers made of equivalent particles (also called homodimers), electromagnetic coupling between the surface plasmon resonances (SPRs) of the individual nanoparticles results in the enhancement of the optically allowed in-phase mode, while the out-of-phase mode is dark due to the cancellation of the equivalent dipole moments. In contrast, the dimers made of non-equivalent nanoparticles (e.g., Ag/Au nanoparticle heterodimers) enables the observation of both in-phase and out-of-phase modes. In addition to optically active nanoparticles made of noble metals, various nanoparticles can be integrated through controlled assembly to form hybrid nanoclusters with multiple functionalities. Near-field coupling between different components in the nanoclusters may also lead to new properties that do not exist in individual nanoparticles.
Some progress has been made in controlling assembly of colloidal nanoparticles into specific clusters. One route is that dimers and clusters can be fabricated by spontaneous aggregation of nanoparticles when tuning the stabilities of colloidal nanoparticles. For instance, citrate-stabilized Au nanoparticles can form aggregated clusters upon addition of HCl to the nanoparticle dispersions to reduce stability of the Au nanoparticles. Amphiphilic nanoparticles capped with both hydrophilic and hydrophobic polymers can also be driven to assemble into dimers by tuning the hydrophobicity of solvents. Due to the absence of specific chemical bonding between the individual nanoparticles in the aggregated clusters, additional encapsulation process with amorphous shells (e.g., SiO2, polymer) is usually necessary to fix the clusters. An alternative approach of for assembled clusters with increased stability relies on the modification of the nanoparticle surfaces with linker molecules that can bind nanoparticles together. In the pioneering work demonstrated by Alivisatos and Mirkin, DNA-modified Au nanoparticles can assemble into dimers, trimmers, and larger structures through the specific hybridization of the complementary single-stranded DNA molecules. Novak and Feldheim have also assembled Au nanoparticles into clusters of dimers, trimmers and tetramers with the use of rigid thiol-functionalized phenylacetylenes as molecular links and templates. Apparently it is difficult to synthesize specific clusters with high yield through this approach: the complete coverage of linker molecules on the surfaces of Au nanoparticles passivates the nanoparticle surfaces due to the strong bonding (usually S—Au covalent bonds) between the linker molecules and the Au surfaces. Such surface passivation leads to the hindrance to deposit other functional molecules on the Au nanoparticles for further applications.
To overcome the limitations, an attractive strategy is developed to selectively decorate the partial surface of a source nanoparticle with synthetic organic or biological molecules to form “binding patches” that can specifically bond with other nanoparticles. The remaining unmodified surface of the source nanoparticle is still active for deposition of interesting species. In order to creating the asymmetric chemistries on the surface of a source nanoparticle, the surface of the nanoparticle has to be partially hidden when it is exposed to linker molecules. One straightforward approach is to deposit nanoparticles on a solid substrate. The physical contacts between the nanoparticles and the substrate prevent the graft of linker molecules on the contact surfaces, leading to formation of “binding patches” only on the regions that exposed. Re-disperse the patched nanoparticles in appropriate solvents and mix with other nanoparticles may facilitate the assembly of them into dimers and high-level clusters with improved yield and purity. However, the ratio of different surface patches in nanoscale is difficult to tune by a cost-effective manner and scalability of this method is limited by the sizes of the solid substrates.